1. Field of the Invention
The present invention relates to a Light Emitting Diode (LED) driving circuit; in particular, to a LED driving circuit driven by constant current.
2. Description of Related Art
Referring to FIG. 1, which shows a prior art LED driving device. The LED driving device comprises a constant current driver 10, a power supply 20, an LED module 30, and a current detecting resistor R. The constant current driver 10 comprises an error amplifier 12 and a transistor switch 14. The power supply 20 provides a driving current flowing through the LED module 30, the constant current driver 10, and the current detecting resistor R. The current detecting resistor R generates a current detecting signal indicating the magnitude of the driving current. An inverting terminal of the error amplifier 12 receives the current detecting signal, and a non-inverting terminal of the error amplifier 12 receives a reference voltage Vr, so that the error amplifier 12 may output a driving signal based on the current detecting signal and the reference voltage Vr. The transistor switch 14 includes a drain D, a source S, and a gate G, wherein the drain D is coupled to the LED module 30, the source S is coupled to the current detecting resistor R, while the gate G receives the aforementioned driving signal. The error amplifier 12 adjusts the level of the driving signal based on the current detecting signal and the reference voltage Vr, so as to modify the equivalent resistance of the transistor switch 14, thus allowing the level of the current detecting signal to be equal to the level of the reference voltage Vr, thereby achieving the objective of constant current.
The power source terminal of the conventional error amplifier 12 is connected to the drain D of the transistor switch 14, and therefore the highest level of the outputted driving signal thereof is the level of the drain D. In order to prevent poor performance in the LED driving device, the level of the drain D should not be overly high, and the maximum load current of the transistor switch 14 is accordingly restricted. In case that the LED driving device needs to be driven with greater current, and the level of the driving signal outputted by the error amplifier 12 is to be limited under the level of the drain D, it is inevitable to enlarge the die size of the transistor switch 14, causing undesirable increase in the cost of the constant current driver.